Just pour some chemicals into the tank one day and kill the fish.
But seriously...not only do I hate ungrateful people, I hate people who can't do a favour for a mate without expecting anything in return. (note: I'm not implying anything to do with the original story in the first post).
We've got this mate of ours that just wont do a single favour without expecting something. He installed my mates headunit in his car...carton of beer. He lends you a trailer....carton of beer. Gives you a hand moving stuff for a few hours...carton of beer. I mean, for God's sake...I thought mates helped each other out.
I've swapped engines and done all sorts of work on mates cars, spent an entire weekend helping another mate move house, polished his BMW M3 for him while he was at work because a guy was coming round to buy it etc etc etc and never once asked for anything in return. That's not to say my mates never showed their appreciation though and I know if I need something from them, they'll give it if they can. Not to mention if I ever got into a big bar fight I know who'd be jumping in for me no questions asked

Except for one of them...not without a carton.